A polarizer (polarizing element) has a function to selectively pass light having a predetermined polarization plane and is widely used in various optical systems. Major fields of use of polarizers include devices for optical communication and liquid crystal displays including projection-type liquid crystal displays. The present invention is a technology applicable to polarizers used in wide areas. A polarizer according to the present invention will be described by focusing on application to a projection-type liquid crystal display to show characteristics characterized particularly in a visible light region.
In recent years, projection-type liquid crystal displays are widely used as display units for displaying on a big screen. Rear projection-type liquid crystal displays are mainly used for big-screen TVs and front projection-type liquid crystal displays for presentation of personal computer data. A projection-type liquid crystal display has a structure to enlarge and project an image on small liquid crystal elements onto a big screen by using an optical system of projection. A detailed technical description can be found, for example, in Non-Patent Document 1 (big-screen display).
FIG. 1 shows a configuration of a typical projection-type liquid crystal display. Light from a light source 4 is separated into blue (B), green (G), and red (R) components by optical components 5 to 16. The separated lights are guided to corresponding liquid crystal elements 2B, 2G, and 2R, respectively. The liquid crystal elements 2R, 2G, and 2B have incident-side polarizers 1R, 1G, and 1B on the incident side and exit-side polarizers 3R, 3G, and 3B on the exit side, respectively. A set of polarizers each having an incident-side polarizer and an exit-side polarizer corresponding to red, green, or blue has a function to selectively allow light passed through the liquid crystal element in a predetermined polarization direction to pass. This function allows lights in three primary colors passed through the liquid crystal elements 2B, 2G, and 2R to become a light intensity modulated image signal. These lights in three primary colors are further synthesized optically by a synthesis prism 17 and further projected onto a screen 19 through a magnifying projector lens system 18.
Polarization characteristics required for a polarizer include a property that transmits optical signals having a desired polarization plane, while at the same time blocking unnecessary optical signals having a polarization plane perpendicular thereto. That is, a desired property is to have a large transmittance with respect to light having a desired polarization plane and a small transmittance with respect to light having a polarization plane perpendicular thereto.
The ratio of these transmittances is called an extinction ratio and is widely used by those skilled in the art as a performance index representing performance of a polarizer. Performance required for polarizers applied to a projection-type liquid crystal display is to have a large transmittance and a large extinction ratio with respect to an optical signal. For a projection-type liquid crystal display, performance required for a polarizer is said to preferably have the transmittance of 70% or more with respect to light of the wavelength to be used and the extinction ratio of 10:1, preferably 3000:1 (Patent Document 1). Values of the transmittance and extinction ratio required for a polarizer are determined depending on a device to which the polarizer is applied.
A social demand for a projection-type liquid crystal display is a demand to realize bigger and clearer images by a smaller device. To realize this demand, a recent technical trend is to apply a light source of a larger quantity of light and to use smaller liquid crystal elements. As a result, light of higher energy density is introduced not only to liquid crystal elements, but also to polarizers placed before and after the liquid crystal elements. Particularly high heat resistance and light resistance are increasingly demanded for polarizers having a function to absorb unnecessary light.
According to principles of polarizers, dichromatic polarizers that selectively absorb light depending on the polarization plane and non-dichromatic polarizers (such as a Brewster polarizer) are known (See Patent Document 2). Dichromatic polarizers have thin elements and do not need any special device to absorb unnecessary light and thus are desired for projection-type liquid crystal displays whose miniaturization is particularly demanded.
Currently, dichromatic polarizers realizing practical optical performance in the visible light region are only polarizing films made of organic material. However, polarizers made of organic resin have a fatal defect of low heat resistance (See Patent Document 1).
To rectify the defect, polarizing films made of organic resin are used by sticking polarizing films to a sapphire substrate having a high thermal conductivity (Patent Document 3). However, the polarization function of polarizers stuck to sapphire having an excellent thermal conductivity may be degraded due to technical requirements of higher intensity in recent years, that is, light absorption/heat generation in a green region with the highest intensity. Thus, a cooling device including a cooling fan is installed in a projection-type liquid crystal display to protect organic resin films from heat. The cooling device not only is against social needs of miniaturization, but also creates another problem of noise.
As a method to solve this technical problem, an idea of applying polarizing glass applied to elements for optical communication to projection-type liquid crystal displays has been proposed (Patent Document 1). However, the invention disclosed in Patent Document 1 does not disclose any technology to provide effective characteristics to glass polarizing elements with respect to light in the visible light region.
Here, the technical background of polarizing glass will be briefly described. As schematically shown in FIG. 2, the polarizing glass is glass characterized in that metallic fine particles 102 having shape anisotropy oriented and dispersed in an optically transparent glass substrate 100 are contained. Polarization characteristics are realized by using an anisotropic resonance absorption phenomenon of surface plasmons present on the surface of the metallic fine particles 102 (See Patent Document 4 and Non-Patent Document 2).
FIG. 3 shows surface plasmon absorption characteristics of metallic fine particles cited from Patent Document 4. Wavelength dependence (solid line) of optical absorbance depending on the polarization plane when light having polarizability is transmitted through glass in which metallic fine particles shown in FIG. 2 are dispersed is shown in FIG. 3. FIG. 2 shows a case in which metallic fine particles have shape anisotropy and, as a special case thereof, metallic fine particles may be spherical having no shape anisotropy. In FIG. 3, surface plasmon absorption of spherical metallic fine particles is shown as a reference state (broken line).
The broken line A in FIG. 3 corresponds to surface plasmon resonance absorption by spherical metallic fine particles. Resonance absorption of metallic fine particles having shape anisotropy shows different characteristics due to correlations between the polarization plane of incident light and metallic fine particles having shape anisotropy. When the polarization plane is in parallel with the longitudinal direction of metallic fine particles, characteristics indicated by B are exhibited. It is seen that the wavelength of resonance absorption is shifted to a longer wavelength as compared with the characteristics A. It is known that this resonance absorption wavelength depends on the ratio of a longer diameter to a shorter diameter of metallic fine particles and the resonance absorption wavelength becomes larger as the ratio increases (See Non-Patent Document 2). With respect to light having the polarization plane perpendicular to the longitudinal direction, on the other hand, properties shown by a solid line C are exhibited. That is, resonance absorption is more exhibited for light of a shorter wavelength than that of the resonance wavelength of spherical metallic fine particles.
From the graph shown in FIG. 3, it is understood that the glass exhibits polarization characteristics with respect to light near 600 nm. That is, the glass has a small transmittance with respect to light having the polarization plane in parallel with the longitudinal direction of metallic particles due to strong absorption. On the other hand, the glass shows poor absorption of light having the polarization plane perpendicular to the longitudinal direction of metallic particles and, therefore, a larger transmittance. As shown in FIG. 2, polarization characteristics are realized by light having a polarization plane perpendicular to the longitudinal direction of metallic fine particles being selectively transmitted through the glass.
Many technologies have been proposed for polarizing glass and glass polarizers using polarizing glass. Many of these technologies relate to glass polarizers applicable to light in the infrared region (such as Patent Document 5 and Patent Document 6) and few technology applicable to light in the visible light region used in a projection-type liquid crystal display, which is an object of the present invention, is disclosed.
Patent Document 7 discloses a technology to provide polarizers effective for light in the visible light region by using characteristics of copper fine particles having shape anisotropy. Characteristics disclosed in Patent Document 7 are shown in FIG. 4. As seen in FIG. 4, a large extinction ratio particularly for wavelengths equal to 600 nm or less cannot be realized. That is, the ratios (extinction ratios) of values of parallel transmittance curves D and F to those of transmittance curves C and E perpendicular to the stretch axis are small and also the value of the transmittance C is only 10 to 30%, leading to a conclusion that the polarizer does not have practical characteristics.
Patent Document 8 discloses a technology to realize dichromatic absorption with respect to wavelengths in the visible light region. However, there is no specific and quantitative description to realize a high transmittance and a high extinction ratio and thus, the technology cannot be considered to be able to realize polarizers. Like Patent Document 8, Patent Document 9 proposes a technology to obtain an effective extinction ratio in the visible light region, but no technology to realize a high transmittance is disclosed.
CODIXX AG offers polarizing glass effective in the visible light region by using a manufacturing technique providing shape anisotropy to silver fine particles by introducing silver ions by diffusion from the glass surface, causing silver fine particles to deposit by heat treatment and stretching the glass (Non-Patent Document 3). However, since the ion diffusion process is generally unstable and concentrations of silver ions are distributed in the thickness direction of the glass, dimensions of generated silver particles tend to be non-uniform. As a result, the ion diffusion process has a weak point of producing fluctuations in characteristics of polarizers.
A different manufacturing method from the above technique of CODIXX AG is used for infrared glass polarizers for communication industrially widely used(Patent Document 4 and Patent Document 5). As schematically shown in FIG. 5, glass in which halogen and silver ions are melted is produced as step 1 (glass production). Next, silver halide fine particles are caused to deposit by heat treatment as step 2 (silver halide deposition). Next, glass in which needle-shaped fine particles of silver halide are oriented and dispersed is produced by stretching glass in which silver halide fine particles are dispersed as step 3 (glass stretch). Lastly, silver fine particles having shape anisotropy are generated by reducing silver halide as step 4 (reduction).
Conventionally, it is understood that polarizers manufactured by this manufacturing method do not exhibit practical performance that can be used for visible light region (Patent Document 5).
FIG. 6 is cited from Patent Document 5 and does not realize performance that is required for a polarizer applicable to projection-type liquid crystal displays. The cause thereof will be described using FIG. 4.
The curve C in FIG. 4 shows that surface plasmon resonance absorption with respect to light having a polarization plane perpendicular to the longitudinal direction of metallic fine particles having shape anisotropy is present at about 350 nm to 400 nm. At the same time, the curve C in FIG. 4 also shows that an influence thereof extends from 500 nm to 600 nm. The influence is that light having a polarization plane to pass through is absorbed. In other words, the transmittance of the light to be transmitted is suppressed. Thus, the transmission curve A in FIG. 6 shows a small transmittance value in the wavelength of 500 nm to 600 nm.
For polarizers applied to light in the infrared region, light to be transmitted has a wavelength far away from the wavelength of the resonance absorption and the above influence is at a negligible level, causing practically no problem. In contrast, when realizing polarizers for visible light, the above influence is at a level that cannot be ignored. Therefore, to realize a polarizer applied to visible light, a new technical means for minimizing light absorption in the wavelength range of 500 nm to 600 nm is needed.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-77850    Patent Document 2: Japanese Patent Application Laid-Open No. 2002-519743    Patent Document 3: Japanese Patent Application Laid-Open No. 2000-206507    Patent Document 4: U.S. Pat. No. 4,479,819    Patent Document 5: Japanese Patent No. 1618477    Patent Document 6: Japanese Patent No. 2740601    Patent Document 7: Japanese Patent No. 2885655    Patent Document 8: Japanese Patent Application Laid-Open No. 2004-523804    Patent Document 9: Japanese Examined Application Publication No. 2-40619    Patent Document 10: Japanese Patent No. 2628014    Patent Document 11: Japanese Patent No. 3549198    Non-Patent Document 1: N. Nishida, “Big-Screen Display (Series, Advanced Display Technology 7)”, Kyoritsu Shuppan, Tokyo, 2002    Non-Patent Document 2: S. Link and M. A. El-Sayed, J. Phys. Chem. B103 (1999), pp. 8410-8426    Non-Patent Document 3: K. Suzuki, Kogyo Zairyo Vol. 52, No. 12, pp. 102-107